1. Field of the Invention
The present invention relates to a process for preparing lacquer polyisocyanates with aliphatically and aromatically bound isocyanate groups, the lacquer polyisocyanate obtained by this process and their use in coating compositions.
2. Description of the Prior Art
Coating compositions based on polyisocyanates are well known and have been disclosed in H. Wagner, H. F. Sarx, Lackkunstharze 5th ed. p. 153 et seq., Carl Hanser Verlag Munich 1971. The lacquer polyisocyanates which are used as cross-linking agents in this type of coating composition may have both aromatically and aliphatically bound isocyanate groups. The lacquer polyisocyanates are prepared in general by the modification of aliphatic and/or aromatic monomeric diisocyanates to form polyisocyanates containing isocyanurate, uretdione, urethane, allophanate and/or biuret groups, followed by removal of excess, unreacted starting diisocyanates, in particular by distillation. Lacquer polyisocyanates based on aliphatic diisocyanates have an exceptional resistance to the effects of light, while those based on aromatic diisocyanates are highly reactive and are used as hardening and cross-linking agents in quick drying two-component polyurethane coating compositions.
The preparation of lacquer polyisocyanates with isocyanurate groups by mixed trimerization of hexamethylene diisocyanate (HDI) and diisocyanatotoluene (TDI) is also known (U.S. Pat. No. 3,645,979 and DE-OS 1,954,093). In this mixed trimerization lacquer polyisocyanates are obtained after distillative removal of excess diisocyanates by thin-film vaporization, which combine the advantages of aliphatic and aromatic lacquer polyisocyanates. One of the disadvantages of the prior art process is that excess starting diisocyanate has to be removed by distillation after completion of the trimerization reaction, which exposes the resulting product to high temperatures that can cause yellowing. Another disadvantage is to be seen in the solid consistency of the resulting mixed trimer.
An object of the present invention is to provide a new process for preparing lacquer polyisocyanates containing isocyanurate groups and having aliphatically and aromatically bound isocyanate groups, in which the costly removal of excess starting polyisocyanate after the trimerization reaction is not required, and which nevertheless leads to monomer-free lacquer polyisocyanates which are liquid at room temperature. The term "liquid lacquer polyisocyanate" also includes liquid solutions of lacquer polyisocyanates, which are solid at room temperature, in lacquer solvents at a solids content of at least 60 wt. %. Since the process according to the invention does not involve costly thin film distillation for removing monomeric starting isocyanates, the production of these products can be undertaken in the presence of solvents, so that at no point in the process according to the invention are solid resins like the products from the previously disclosed prior art.
This object may be achieved with the process according to the invention described in detail below. This process is based on the surprising observation that aromatic isocyanates, in particular TDI, can be selectively trimerized in the presence of high concentrations of aliphatic isocyanate groups down to a residual content of aromatic diisocyanates of less than 0.5 wt. % to obtain virtually toohomer-free liquid lacquer polyisocyanates having aliphatically and aromatically bound isocyanate groups and an NCO content of 5 to 25 wt. %.